During the summer of 1943, a sixth-order excitation caused several turbine-blade failures. Franz resorted to asking a professional musician to stroke the blades with a violin bow and use his trained musical ear to determine the ringing natural frequency. The air ministry, however, was getting increasingly impatient and scheduled a conference in December 1943. Bentele attended the conference and listened to the numerous arguments pertaining to material defects, grain size, and manufacturing tolerances. When his turn came, Bentele told the assembled group that the culprits were the six combustor cans and the three struts of the jet nozzle housing after the turbine. These induced forced excitation on the turbine rotor blades where a sixth-order resonance occurred with the blade-bending frequency in the upper speed range. The predominance of the sixth-order excitation was due to the six combustor cans (undisturbed by the 36 nozzles) and the second harmonic of the three struts downstream from the rotor. In the 004A engine, this resonance was above the operating speed range, but in the 004B it had slipped because of the slightly higher turbine speed and the higher turbine temperatures. The problem was solved by raising the blade's natural frequency—increasing blade taper, shortening the blades by 1 millimeter, and reducing the operating speed of the engine from 9,000 to 8,700 rpm. -------- Can anyone confirm this?
I can find no mention of this in Vol 1 of Smith & Creek's 4-volume 'Me262' history ( Classic Publications, 1997 ) but of course, more research may have come to light since then. I can understand the RLM getting impatient at that time however - the 262 had been demonstrated to Hitler at Insterburg airfield on November 26th. This was the famous incident where Hitler exclaimed ' I'm not interested in this aircraft as a fighter - can it carry bombs?......At last ! This is the aircraft I have been demanding for years ! Here it is, but nobody recognised it !'
